NFSv4: Fix locking in pnfs_generic_recover_commit_reqs

The use of the inode->i_lock was converted to a mutex, but we forgot
to remove the old inode unlock/lock() pair that allowed the layout
segment to be put inside the loop.

Reported-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Fixes: e824f99a ("NFSv4: Use a mutex to protect the per-inode commit...")
Cc: stable@vger.kernel.org # v4.14+
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>